More than 30 years ago  it was stated: “The structure of apatite seems to be remarkably stable, permitting a number of rather unusual types of substitution and involving a considerable number of ions.” Crystallochemical investigations during subsequent years have extended the list of isomorphic substituents both qualitatively and quantitatively. However, it should be indicated that one new fundamental concept resulted from the investigations: Crystal chemical considerations do notrequire the balancing of electrostatic charges in such a way that a carbonate apatite can be written as xCa3(PO4)2• yCaCO3, but must involve summation of the charges of all cations (Ca, P and C) versus those of the correct number of oxygens and fluorines which the structure will accommodate. Stating this proposition slightly differently, a carbonate apatite is not a mixture in any sense of tricalcium phosphate and calcium carbonate, but an individual structure [A10(ZO4)6X2] with which its composition must be reconciled in respect to such physical properties as density and refractive index.
KeywordsCoordination Number Crystal Chemistry Tricalcium Phosphate Carbonate Apatite Apatite Structure
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